An optical amplifying apparatus for directly amplifying a light signal has been now developed in a variety of technical fields such as an optical communication system. Recently, an optical amplifying apparatus using an optical fiber amplifier, in which rare-earth element or the like is doped to an optical fiber, has been proposed. The optical fiber amplifier amplifies a light signal when a predetermined excitation light is supplied thereto. One kind of such an optical amplifying apparatus is described in UK Patent Publication "GB-2175766A".
In such an optical amplifying apparatus, a noise light occurs in an optical direct amplifier because of the amplifying theory of the amplifier, the structural reason and the like. For the purpose of monitoring operating condition of the optical amplifier, a conventional optical amplifying apparatus is provided with a detector for detecting noise occurring in the amplifier.
A conventional optical amplifying apparatus includes an optical direct amplifier for amplifying an input signal light directly, an optical (or beam) splitter for splitting the amplified light, and an optical filter for detecting a noise light occurring in the optical amplifier. The optical filter has a bandstop function in that a signal light having a narrow wavelength band is attenuated thereat and a noise light having a relatively wide wavelength band passes therethrough.
In the conventional optical amplifying apparatus, an S/N ratio of an output signal is determined in accordance with an amount of the detected noise light.
According to the conventional optical amplifying apparatus, however, there is a disadvantage in that the optical filter must have high performance, because it is required to detect only a noise light with high precision.